

Understanding Efficiency of Switched Capacitor DCDC Converters for BatteryPowered ApplicationsBy Sanjeevi Thirumurugesan, Vidatronic Abstract Switched Capacitor (SC) DCDC converters are DCDC switching regulators that use only capacitors and switches to transfer charges between the input and the output. This architecture is an alternative to inductorbased DCDC converters that provide several advantages including better ondie integration (capacitors store 10 to 100 times more energy per volume than inductors), low Electro Magnetic Interference (EMI), and lower cost. These characteristics are of particular importance in batteryoperated Internet of Things (IoT) applications where devices with high efficiency, low cost, and small footprint are a necessity. This application note provides a brief theory on the efficiency in SC DCDC converters and a comparative efficiency analysis between the two types of switched converter architectures using a typical application case. Introduction The Switched Capacitor (SC) DCDC converter is a DCDC switching regulator that has been gaining popularity over LDOs and inductorbased switching convertors in applications where high efficiency is desired in a small, integrated system solution. SC DCDC converters use only capacitors as chargetransfer devices. The inductorless power transfer provides multiple advantages over inductorbased switching regulators including fast transient response and reduced system size. Capacitors have better energy density and simpler, more costeffective integration ondie in CMOS processes without additional fabrication steps. These advantages make SC DCDC converters an attractive option for Internet of Things (IoT) applications where low cost and smaller devices are the norm. Using only capacitors as charge transfer devices has its disadvantages as well. In inductorbased switching converters, the charge is stored and transferred in the form of inductor current which enables more efficient control of the output voltage. In SC DCDC converters, voltage control is achieved only with a resistive loss or topology switching, which introduces increased complexity. Supporting a wide supply range and a wide programmability of output voltages is highly desirable in IoT applications, which are predominantly batteryoperated. A common case is a converter supplied by a Liion battery where the battery voltage can vary between 3.4 V and 4.3 V based on the charge or discharge state of the battery. Energy harvesting systems, commonly seen in IoT solutions, also involve widely varying supply voltages to the DCDC converter. The following sections detail how requiring DCDC converters to operate within a wide range of input voltages and support output voltage programmability affects overall efficiency and the tradeoffs involved in achieving a higher efficiency. Theoretical Model The SC DCDC converter can be modelled as a transformer with an ideal conversion ratio and a series resistor, RS. SC DCDC converters can be broadly classified into two types:
Figure 1. Theoretical Model – Single Topology
Figure 2. Theoretical Model – Multiple Topology Efficiency Comparison Between Single And Multiple Topology Converters Single Topology Switched Capacitor DCDC Converter Each SC DCDC converter topology has an ideal voltage conversion ratio (iVCR). This iVCR is the maximum ratio between the output voltage and the supply voltage of the conversion block. In practice, this iVCR is the upper bound for the actual VCR and the converter can only operate at a theoretical efficiency of 100% when this iVCR is met. In SC DCDC converters with a single conversion ratio of iVCR, the theoretical maximum efficiency that can be achieved is given by: This means that for other conversion ratios required due to changes in supply and output programmability, the efficiency suffers. Table 1 shows estimated efficiencies of a 1.8 V nominal output single topology converter supplied by a battery with its voltage varying from 3.4 V to 4.3 V. The efficiency suffers about 20% as you move further away from iVCR towards smaller VCRs. This is worsened to about 30% in cases where the output voltage needs to be programmable as this introduces a wider range of VCRs to cover. A 5/9 topology is chosen for the nonprogrammable converter and a 5/8 topology is chosen for the programmable converter in this application case. Table 1. Efficiency of a single topology switched capacitor DCDC converter Multiple Topology Switched Capacitor DCDC Converter By switching between various topologies based on the required VCR, a multiple topology SC DCDC converter maintains better efficiency over the full range of converter supply and output voltages. The VCR at which to switch to a different topology can be predetermined based on the estimated drop in efficiency below a desired minimum level. Having more topologies with closely spaced iVCRs achieves higher minimum efficiency of the SC DCDC converter. Table 2 shows estimated efficiencies for the 1.8 V output DCDC converter using a 4toplogy SC DCDC converter. Figure 3 is the corresponding depiction of this architecture’s efficiency in graphical form. Table 2. Efficiency of a multiple (4) topology switched capacitor DCDC converter Figures 3 and 4 show the efficiency of the same system under a 4topology and 2topology SC DCDC converter architecture. It is clear that quicker switching to a different iVCR topology as the required conversion ratio moves farther away from the ideal ratio for that topology helps in maintaining a higher minimum system efficiency.
Figure 3. Efficiency of 4Topology Switched Capacitor DCDC Converter
Figure 4. Efficiency of 2Topology Switched Capacitor DCDC Converter Efficiency Tradeoffs The tradeoffs to achieve this high SC DCDC converter efficiency are the increased area due to additional switches needed for switching between multiple topologies and the control logic required to implement it. For this application case, Vidatronic estimates that the 4toplogy SC DCDC converter can be as much as 25% larger than a single topology SC DCDC converter and required design resources will be about 50% higher. Table 3. Tradeoff Summary Summary Since SC DCDC converters are a great fit for the everbroadening array of IoT fields where batteryoperated devices dominate, it is necessary to understand the efficiency tradeoffs inherent in these converters. Based on the end application’s priorities, the customer can choose between single topology SC DCDC converters and various multiple topology SC DCDC converter architectures. For more information on Vidatronic’s Switched Capacitor DCDC converters and other IP cores, go to: http://www.vidatronic.com/ipsolutions/ or contact Vidatronic at sales@vidatronic.com to learn how we can provide the best Switched Capacitor DCDC converter IP for your application. Further Reading
List of Abbreviations
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